Magnetic core-shell dendritic mesoporous silica nanospheres anchored with diamine as an efficient and recyclable base catalyst

Article abstract of DOI:10.1039/d0nj04822d

In the present study, diamine-functionalized magnetic core-shell dendritic mesoporous silica nanospheres have been successfully synthesized by an oil-water biphasic stratification-coating strategy. The shape, size and morphology of the synthesized magnetic nanocatalyst could be characterized by various physicochemical techniques such as, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The characteristic information about the successful immobilization of various functionalities on the nanospheres could be obtained with the help of X-ray powder diffraction (XRD) patterns, Fourier transform-infrared spectroscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX) and thermo-gravimetric analysis (TGA). The details about the magnetic behaviour and surface area of the nanocatalyst could be acquired by vibrating sample magnetometry (VSM) and BET surface analysis, respectively. The synthesized diamine-functionalized magnetic nanoparticles were then explored as a highly efficient catalyst for the Knoevenagel condensation and one-pot synthesis of polyhydroquinolines using aromatic/heteroaromatic aldehydes and aliphatic aldehydes with active methylene compounds under very mild conditions. The synthesized magnetic core-shell dendritic mesoporous silica nanospheres had large surface areas. This large surface area and pore volume could facilitate a proper interaction and penetration of the reactant molecules with the basic amine groups present on the dendritic mesoporous silica nanospheres. The supported nanocatalyst revealed no sign of leaching of the amine groups present inside the dendrimers and therefore, could be reused up to nine times without any noteworthy loss in catalytic activity.

Full text of DOI:10.1039/d0nj04822d

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Magnetic core–shell dendritic mesoporous
silica nanospheres anchored with diamine
Cite this:DOI: 10.1039/d0nj04822d
as an efficient and recyclable base catalyst†
Surabhi, Javaid Shabir, Padmini Gupta, Digvijay Sah and Subho Mozumdar
*
In the present study, diamine-functionalized magnetic core–shell dendritic mesoporous silica nanospheres
have been successfully synthesized by an oil–water biphasic stratification-coating strategy. The shape,
size and morphology of the synthesized magnetic nanocatalyst could be characterized by various
physicochemical techniques such as, field emission scanning electron microscopy (FE-SEM) and
transmission electron microscopy (TEM). The characteristic information about the successful
immobilization of various functionalities on the nanospheres could be obtained with the help of X-ray
powder diffraction (XRD) patterns, Fourier transform-infrared spectroscopy (FT-IR), energy dispersive
X-ray spectroscopy (EDX) and thermo-gravimetric analysis (TGA). The details about the magnetic
behaviour and surface area of the nanocatalyst could be acquired by vibrating sample magnetometry
(VSM) and BET surface analysis, respectively. The synthesized diamine-functionalized magnetic
nanoparticles were then explored as a highly efficient catalyst for the Knoevenagel condensation and
one-pot synthesis of polyhydroquinolines using aromatic/heteroaromatic aldehydes and aliphatic
aldehydes with active methylene compounds under very mild conditions. The synthesized magnetic
core–shell dendritic mesoporous silica nanospheres had large surface areas. This large surface area and
pore volume could facilitate a proper interaction and penetration of the reactant molecules with the
basic amine groups present on the dendritic mesoporous silica nanospheres. The supported nanocata-
lyst revealed no sign of leaching of the amine groups present inside the dendrimers and therefore, could
be reused up to nine times without any noteworthy loss in catalytic activity.
Received 30th September 2020,
Accepted 11th November 2020
DOI: 10.1039/d0nj04822d
rsc.li/njc
8
9
10
solid support materials such as, Al O , MCM-41, MCM-48,
2
3
12
1
. Introduction
11
13
SBA-15, hollow mesoporous carbon, graphene oxide, and
In the last few years, different types of homogeneous base MWCNTs have been utilized to heterogenize the homogeneous
catalysts have been utilized on a large scale to catalyze different catalysts. It has been observed that silica-based materials
14
1
kinds of organic reactions. Many reports on homogeneous possesses great potential for surface modifications because of
base catalysts have been published, where different types of their larger surface area, nanometre size, large number of silanol
2
3
15–17
homogeneous catalysts such as piperidine, chitosan, ionic groups, high mechanical and thermal stability.
But there are
4
5
6
liquids, pyrrolidine, and K HPO have been utilized as base some disadvantages associated with the silica-based materials,
2
4
catalysts. Homogeneous catalysis is known for its marvellous such as the tedious method of its separation from the reaction
catalytic activity and selectivity, but the problem associated mixture such as filtration and centrifugation. Mesoporous silica
with homogeneous catalysis is that, it suffers from difficulties materials, such as MCM-41, and SBA-15 have tubular pores,
in the separation of the catalyst from the reaction mixture which lead to the blocking of pores on covalent immobilization
7
18
resulting in poisoning of the final product. To overcome this of organosilane derivatives. This ultimately reduces the accessi-
difficulty, the scientific community has utilized various solid bility of active sites for the reactant molecules and consequently
support materials for the immobilization of basic catalytic active leads to the decrease in the catalytic efficiency of the
1
9–22
moieties either covalently or non-covalently. Different types of nanocatalyst.
To overcome this complexity, different types
23
of silica-based fibrous materials were reported such as KCC-1,
dendritic SiO2, and cSiO @DFNS. These materials have
fibrous morphology with high surface area, large pore size and
pore volume and due to this the chances of pore-blocking
significantly reduces. A drawback associated with fibrous silica
24
25
2
Department of Chemistry, University of Delhi, 110007, India.
E-mail: subhoscom@yahoo.co.in
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04822d
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Different studies have been published for the one-pot synthesis
4
2
of polyhydroquinolines such as Fe
3
O
4
@SiO
2 2
/ZnCl ,
DBH or
43
44
DCH, and MMWCNTs-D-(CH ) -SO H. DHPQs are a special
2
4
3
class of heterocyclic system owing their remarkable pharmacolo-
gical properties; they are also known as calcium channel blockers.
4
5
46
The DHPQ derivatives are used as antidiabetic, antitumor,
47
48
49
antitherosclerotic, bronchodilator, and geroprotective agents.
The structures of few biologically active polyhydroquinolines
are given in Fig. 1.
In continuation of attempts to synthesize nanostructured
5
0,51
materials for numerous organic transformations,
the pre-
sent study reports the synthesis of diamine-functionalized
magnetic core–shell dendritic mesoporous silica nanospheres
3 4 2 2 2
(Fe O @SiO @DSiO –NH ). The prepared nanocatalyst turned
out to be a highly efficient, stable, eco-friendly, magnetically
recoverable, low cost and heterogeneous catalyst for the
Knoevenagel and Hantzsch condensations. The synthesized
nanocatalyst showed excellent catalytic activity towards both
the reactions and was efficiently utilized for a number of
catalytic cycles without any noticeable loss in catalytic activity.
Fig. 1 Structures of few bioactive polyhydroquinolines.
is the difficulty in separating it from the reaction mixture. The
difficulty in the separation of dendritic silica could be solved
by supporting it on a material that could be easily recovered 2. Experimental
3 4
without the tedious method of filtration and centrifugation. Fe O
2.1 Materials
magnetic nanoparticles gained more attention due to their large
surface area, easy availability, low toxicity and their ability to be
Fe (SO and FeSO
2
4
)
3
4
Á7H
2
O were purchased from Sigma-Aldrich.
26
Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium
chloride (CTAC) and cyclohexane were purchased from Sigma-
Aldrich, whereas 3-(2-aminoethylamino) propyltrimethoxysi-
lane was purchased from Alfa Aesar. All the other chemicals
were purchased from Thomas Baker, Spectrochem and Merck.
All the reagents were used without any further purification.
easily recycled using an external magnetic field. Therefore,
Fe O magnetic nanoparticles could be extensively used as
3
4
solid support materials in designing heterogeneous catalysts.
Many reports have been published where magnetic nanoparticles
3 4
were utilized as solid support materials such as Fe O @MCM-41-
2
7
28
29
SO
FGT, Fe
SiO –OSO
3
H, SBA-15@Fe
3
O
4
@Isa, Fe
3
O
4
@mSiO
H, Fe @SiO
-IL. These nanocatalysts were
2
–NH
2
,
nano-
3
0
31
32,33
3
O
4
@D-NH-(CH
2
)
4
-SO
3
3
O
4
2
Fe O @
3 4
2.2 Synthesis of magnetic nanoferrites
3
4
35
2
3
H, and Fe
3
O
4
@SiO
2
The Fe nanoparticles were synthesized by a previously
3 4
O
utilized for various organic transformations such as Knoevena-
gel condensation, Michael addition, Hantzsch condensation
and N-formylation of amines. In order to overcome the above
said limitations, the development of highly efficient nanocata-
lysts having high surface area, large pore volume and easy
recoverability is still currently in demand.
5
2
reported co-precipitation method. In a typical procedure,
Fe (SO ) (3.0 g) and FeSO Á7H O (2.1 g) were dissolved in
2
4 3
4
2
distilled water (160 mL) and the resulting mixture was vigor-
ously stirred for an hour until the solution became orange in
colour. Furthermore, the pH of the reaction mixture was main-
4
tained at 10 by dropwise addition of NH OH (25%). On addition
Knoevenagel condensation is an important reaction for the
3
6
of ammonium hydroxide, the colour changes instantaneously
from orange to black. The reaction mixture was stirred con-
tinuously for two hours at 60 1C. The black coloured precipi-
tates of magnetic nanoparticles were separated by using an
external magnet. The obtained black precipitate was washed
several times with distilled water and ethanol until the pH of
generation of C–C bonds. This is a nucleophilic addition
reaction between aromatic aldehydes and active methylene
compounds, which results in the production of various phar-
3
7
maceutical compounds. There are several reports that have
been published for the synthesis of Knoevenagel condensation
3
8
3 4 2
products including Fe O @SiO @PAMAM-G2. The products
3 4
the reaction mixture became neutral. The obtained Fe O
nanoparticles were dried at 60 1C in vacuum for 6 h.
obtained from Knoevenagel condensation are used as inter-
mediates for the synthesis of many heterocyclic compounds of
medicinal importance.
2
.3 Synthesis of silica-coated magnetic nanoferrites
Silica-coated magnetic nanoferrites were synthesized by using
The first synthesis of biologically active 1,4-dihydropyridines the sol–gel method. In a typical procedure, Fe O nanoparticles
On the other hand, multicomponent reactions (MCRs) play a
3
9,40
vital role in the formation of many C–C and C–N bonds.
3
4
(
DHPQs) was reported by Arthur Hantzsch in 1881 by four (1.0 g) were dispersed in ethanol (250 mL) followed by dropwise
component reactions between aromatic aldehydes, active methy- addition of NH OH 25% (6.0 mL) and TEOS (2.0 mL). The
brownish precipitates were collected after constant stirring for
4
41
lene compounds, ammonium acetate and ethylacetoacetate.
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24 h. The precipitates were then washed with distilled water
and ethanol multiple times and dried at 60 1C in a vacuum
for 6 h.
2.4 Synthesis of magnetic core–shell dendritic mesoporous
silica
The dendritic mesoporous silica was synthesized on the surface
of silica-coated magnetic nanoferrites by using an oil–water
biphasic stratification method. In this typical procedure, 0.2 g
of silica-coated magnetic nanoferrites was dispersed in a mix-
ture containing CTAC (24 mL) and H O (36 mL), followed by the
2
addition of TEA (0.18 g), and then the resulting mixture was
ultrasonicated for about an hour. Afterwards, the reaction
mixture was maintained at 60 1C for about another hour.
Subsequently, a mixture of cyclohexane (20 mL) and TEOS
(4 mL) was added dropwise to the reaction mixture, while
stirring at 60 1C. The reaction mixture was stirred continuously
under reflux conditions at 60 1C in an oil bath for 12 h. The
product was extracted using an external magnet. The product
was washed several times with ethanol and distilled water.
Then, the collected solid residue was dried under vacuum at
6
6
0 1C for approximately 6 h. Later on, calcination was done at
00 1C to remove the template i.e. CTAC surfactant and the
Scheme 1 Schematic diagram of the stepwise synthesis of the Fe
SiO @DSiO –NH nanocatalyst.
3 4
O @
2
2
2
hydrophobic organic phase. Finally, the magnetic core–shell
dendritic mesoporous silica nanospheres were obtained.
added in water (2 mL). The reaction mixture was ultrasonicated
for 10 minutes and after that the reaction mixture was stirred at
2
.5 Synthesis of diamine-functionalized magnetic–core shell
dendritic mesoporous silica (Fe @SiO @DSiO –NH
3
O
4
2
2
2
)
35 1C. The progress of the reaction was monitored by TLC. The
For the synthesis of diamine-functionalized magnetic– solid product was extracted with ethyl acetate. The catalyst was
core shell dendritic mesoporous silica, magnetic core shell recovered by using an external magnet. The pure products were
dendritic mesoporous silica (500 mg) was stirred with excess obtained after recrystallization using hot ethanol.
of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (762 mL) in
toluene (250 mL) at 80 1C for 6 h. The solution was filtered and
the residue was thoroughly washed with toluene (3 Â 20 mL),
and then with ethanol (3 Â 20 mL) and ultimately the solid The FT-IR spectra of the stepwise synthesized nanocatalyst i.e.
product was air-dried. Fe O @SiO @DSiO –NH were recorded using a PerkinElmer
3
. Characterization techniques
3
4
2
2
2
Spectrum RXI-Mid Infra-red spectrometer in the range of
2.6 Synthesis of the Knoevenagel condensation product
À1
4000–400 cm
through the KBr pellet method. Powdered
In a typical procedure for Knoevenagel condensation, aldehyde X-ray diffraction (XRD) was recorded at a scanning rate of
À1
(1 mmol) and active methylene compound (1 mmol) were 41 min
using 3 kW radiation on a Bruker D8 Discover
added to a round bottom flask containing distilled water diffractometer in the range of 51 to 801. The TG Analysis was
o
(
3 4 2 2 2
2 mL) followed by addition of Fe O @SiO @DSiO –NH nano- done at a temperature range of 50 C–850 1C by using a Pyris
catalyst (5 mg). The mixture was magnetically stirred at room Diamond TGA-DTA instrument. Through this method, the
temperature for an appropriate time. The progress of the changes in the chemical and physical properties of the synthe-
reaction was monitored by thin layer chromatography (TLC). sized nanomaterial were analyzed at a constant heating rate
Following the completion of the reaction, the nanocatalyst was throughout the process. Scanning electron microscopy (SEM)
recovered by using an external magnet and washed with and transmission electron microscopy (TEM) provided the
ethanol and dried for further use. The product of the reaction information about morphology, shape, size and structure of
was extracted with DCM. The filtrate was removed by a rotary the nanocatalyst by using the TALOS model. An energy dis-
evaporator to obtain the solid product. The pure product was persive X-ray (EDX) spectroscopic technique was utilized to
obtained after recrystallizing in absolute ethanol.
determine the elemental composition of the synthesized nano-
materials. BET surface analysis was done with a Quantachrome
2
.7 One-pot synthesis of polyhydroquinolines
1
13
Instrument, ASI-CI-11 model. The H and C NMR spectra
In a typical synthesis of polyhydroquinolines, aldehyde of the Knoevenagel and Hantzsch condensation products
1 mmol), dimedone (140.18 mg), ethyl acetoacetate (128 mL), were obtained using 400 MHz and 100 MHz Jeol JNM-EXCP
(
ammonium acetate (77.8 mg) and the catalyst (10 mg) were 400 instruments, respectively.
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Fig. 3 Magnetization curves (a) Fe
O
3 4
,
(b) Fe
3 4 2 3 4
O @SiO , (c) Fe O @
SiO @DSiO and (d) Fe @SiO @DSiO
2
2
3
O
4
2
2
–NH
2
.
3 4 3 4 2 3 4 2 2
Fig. 2 FT-IR spectra of (a) Fe O , (b) Fe O @SiO , (c) Fe O @SiO @DSiO
3 4 2 2 2
and (d) Fe O @SiO @DSiO –NH .
Si–OH groups, which could be further used for the functional
modification of diamine groups over it. The spectrum shown by
Fig. 2(d) is for diamine-functionalized dendritic mesoporous
4
. Results and discussion
À1
silica nanospheres. The absorption band at 1017 cm could
À1
The diamine-functionalized magnetic core shell dendritic correspond to Si–O stretching and the peak at 2924 cm could
mesoporous silica nanospheres were synthesized in a stepwise be due to –CH stretching. The new absorption band at
2
À1
manner as described in Scheme 1. The Fe O nanoparticles 1589 cm could be due to the characteristic peak of the N–H
3
4
À1
could be synthesized by the previously reported co- bending mode and the peak at 3442 cm could be attributed to
5
2
precipitation method in an aqueous medium. Following that, the broad peak of N–H stretching. These observations could
silica-coated magnetic nanoparticles could be easily prepared confirm the successful modification of diamine derivatives on
5
3
by the previously reported sol–gel method. Furthermore, the surface of magnetic core shell dendritic mesoporous silica.
silica-coated magnetic nanoparticles could be modified with
The magnetization curves of the synthesized nanocatalysts
dendritic mesoporous silica by an oil–water biphasic coating are shown in Fig. 3. Based on the results, it could be clearly
5
4
strategy. Subsequent to successful synthesis of magnetic observed that the synthesized nanocatalyst was magnetic in
core–shell dendritic mesoporous silica, the surface of this nature. The saturation magnetization value of Fe O4 nano-
3
À1
material could be further modified by functionalization with a particles was found to be approximately 51.0 emu g
as
base through covalent bond formation between surface hydroxyl represented in Fig. 3(a). Following the silanation of the ferrite
groups and organosilane derivatives as shown in Scheme 1.
nanoparticles, a decrease in the saturation magnetization of
The FT-IR spectra of the synthesized nanocatalysts are silica-coated nanoparticles could be observed and this could be
shown in Fig. 2. Firstly, the spectrum represented in Fig. 2(a) due to the successful coating of a silica layer on the surface of
is for the magnetic nanoferrite particles, where the absorption Fe O nanoparticles. The value of saturation magnetization
3
4
À1
À1
band at 582 cm could be due to the Fe–O vibration and the also decreased up to 34.36 emu g as shown in Fig. 3(b).
broad peak at 3404 cm could be due to the surface hydroxyl Followed by the introduction of the dendritic mesoporous silica
À1
groups present on the nanoparticles. The spectrum represented over the surface of silica-coated Fe O particles (Fe O @
3
4
3 4
by Fig. 2(b) is for silica-coated magnetic nanoferrite particles SiO @DSiO ), there was a slight decrease in the magnetization
2
2
À1
À1
where the three additional bands at 1081 cm , 962 cm and of dendritic mesoporous silica nanospheres and then satura-
01 cm could probably correspond to Si–O–Si antisymmetric tion magnetization value obtained was 33.46 emu g
À1
À1
8
as
stretching, Si–O symmetric and Si–O–Si symmetric modes, represented in Fig. 3(c). In the final step of the synthesis
respectively. This confirms the successful coating of the silica protocol, i.e. after the successful immobilization of the diamine
layer on the surface of magnetic nanoparticles. The spectrum derivative on the surface of dendritic mesoporous silica, there
represented by Fig. 2(c) is for dendritic mesoporous silica- was a rapid decrease in the saturation magnetization value and
À1
coated magnetic nanoparticles in which the characteristic it was found to be 17.20 emu g as shown in Fig. 3(d).
À1
À1
À1
bands peaking at 466 cm , 799 cm and 1085 cm could
The energy dispersive spectroscopic analysis could be uti-
be attributed to the Si–O–Si plane bending, Si–O symmetric lized to determine the chemical composition of the prepared
vibrations and Si–O asymmetric vibrations; respectively. The Fe O @SiO @DSiO –NH nanocatalyst. The EDX spectrum shows
3
4
2
2
2
À1
intense peak at 3386 cm could be due to the large number of that it contains various elements such as, carbon, nitrogen, iron,
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3 4 2 2 2
Fig. 4 EDAX Spectrum of Fe O @SiO @DSiO –NH .
silicon and oxygen and their corresponding peaks are shown in was found that the synthesized nanomaterial contained around
Fig. 4. The percentage composition of the corresponding 17.84% carbon, 4.35% hydrogen and 7.04% nitrogen.
elements present in Fe
3
O
4
@SiO
2
@DSiO
2
–NH
2
could be deter-
The morphology and the structural features of the synthesized
mined from the CHN analysis. Based on the CHN analysis, it magnetic–core shell dendritic mesoporous silica nanospheres
Fig. 5 FE-SEM images of (a) Fe
3
O
4
@SiO
2
@DSiO
2 3
, (b) Fe O
4
@SiO
2
@DSiO
2
–NH
2
, (c) TEM images of Fe
3
O
4
@SiO
2
@DSiO
2
at 200 nm and (d) at 50 nm
magnification.
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Fig. 6 (a) TG analysis of Fe
3
O
4
@SiO
2
@DSiO
2
and Fe
3
O
4
@SiO
2
@DSiO
@SiO @DSiO
2
–NH
2
. (b) X-ray diffraction pattern of Fe
3 4 2 2 3 4 2
O @SiO @DSiO and Fe O @SiO @
DSiO
2
–NH
2
, (c–d) N
2
adsorption–desorption curves of Fe
3
O
4
2
2
and Fe @SiO @DSiO –NH , respectively, and pore size distributions
3
O
4
2
2
2
(insets).
could be analysed from the TEM images (Fig. 5(c and d)). Based be due to the destruction of the organic structure on the surface
on the TEM images it could be observed that the synthesized of the dendritic mesoporous silica. No weight loss could be
nanomaterial had a magnetic core surrounded by non-porous observed after 600 1C (Fig. 6(a) red line curve). On comparing
silica, with mesoporous dendritic silica on the outermost sur- the above results, it could be concluded that more organosilane
face. Moreover, the surface of the silica layer was uniform and derivatives have been successfully grafted due to the large pore
monodispersed in nature. FE-SEM images of Fe O @SiO @ size and dendritic structure, as shown by the red line curve.
3
4
2
2 2
DSiO –NH revealed that the nanoparticles had spherical Hence, it could be inferred that the thermal stability of the
morphology as shown in Fig. 5(a and b).
prepared nanocatalyst was excellent.
In the TG analysis, a small amount of weight loss could be
3 4 2
The powdered X-ray diffraction pattern of Fe O @SiO @
2 3 4 2 2 2
observed in both curves below 140 1C, which could be due to DSiO and Fe O @SiO @DSiO –NH are shown in Fig. 6(b).
the evaporation of physically adsorbed water molecules on the Both the XRD pattern contains broad peaks in the 2y range of
surface of the nanocatalysts. Fig. 6(a) shows that in the case of 101 to 271 could indicate the presence of an amorphous silica
Fe O @SiO @DSiO nanospheres, no further weight loss could layer throughout the magnetic nanoparticles. Furthermore, it
3
4
2
2
be observed below 600 1C. This could demonstrate that the could be clearly seen in the diffraction patterns of both the
magnetic core shell dendritic mesoporous silica does not break samples that they possess the characteristic diffraction peaks at
below 600 1C and remains undamaged. Furthermore, as evident 2y values of 30.11, 35.51, 42.31, 53.41, 58.71 and 62.11. These
from the spectrum of Fe
3
O
4
2 2 2
@SiO @DSiO –NH a weight loss of peaks are attributed to corresponding planes i.e. (220), (311),
17.0% could be seen between 140 1C and 600 1C and this could (400), (422), (511) and (440), respectively, shown by the green line.
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Table 1 Optimization study for the Knoevenagel condensation
Fig. 6(b) (pink line). From these observations, it could be
inferred that the developed nanomaterial have a face centred
cubic structure (FCC) and inverse spinel structure. It remained
the same (i.e. inverse spinel structure) even after incorporation
of the base on the surface of dendritic mesoporous silica.
The
SiO @DSiO
conducted to determine the surface area and pore size of the
N
2
adsorption–desorption isotherms of Fe
3 4
O @
2
2
are shown in Fig. 6(c). These investigations were
Temp Time Isolated prepared nanocatalyst. The BET curve, clearly shows that
Entry
1
Catalyst (mg)
Fe @SiO
Solvent
Ethanol
(1C)
(min)
yield %
3 4 2 2
Fe O @SiO @DSiO possesses the dendritic mesoporous silica
channels with uniform nanoscopic space between dendrimers.
The BET surface area and pore volumes of the magnetic core
shell dendritic mesoporous silica nanospheres was found to be
3
O
4
2
@
50
40
80
DSiO –NH
Fe @SiO
DSiO –NH
Fe @SiO
DSiO –NH
Fe @SiO
DSiO –NH
Fe @SiO
DSiO –NH
Fe @SiO
DSiO –NH
Fe @SiO
DSiO –NH (10)
Fe @SiO
DSiO –NH
@SiO –NH
@SiO
DSiO (5)
Blank
2
2
(5)
2
3
4
5
6
7
8
9
3
O
4
2
@
2
(5)
CH
3
CN
50
50
80
25
25
25
50
40
40
40
20
7
92
78
2
2
À1
3
À1
3
O
4
2
@
2
(5)
DCM
THF
571.30 m g and 0.954 cm g ; respectively. The pore size
was found to be about 8.52 nm, this could be proved by the pore
size distribution plot (inset Fig. 6(c)). This could be attributed
to the highly porous structure of dendritic mesoporous silica.
Based on these results, it could be said that the availability of
space between the dendrimers was more and this could help
further in the functionalization with the amine groups. On
immobilization of diamine functionalities over the dendritic
mesoporous silica nanospheres, the surface area decreases
2
3
O
4
2
@
2
(5)
45
2
3
O
4
2
@
2
(3)
H
2
H
2
H
2
O
O
O
75
2
3
O
4
2
@
2
(5)
99
2
3
O
4
2
@
20
40
99
2
2
3
O
4
2
@
2
(5)
Neat
Trace
2
2
À1
Fe
3
O
Fe
4
2
2
2
@
(5)
H
H
2
O
O
40
40
40
40
85
30
from 571.0 to 197.0 m
g ; the pore diameter decreases
1
0
3
O
4
2
slightly from 8.52 to 6.58 nm and pore volume from 0.954 to
2
3
À1
0
.658 cm g as shown in Fig. 6(d). This could authenticate the
1
1
H
2
O
40
40
10
successful incorporation of basic moieties over the dendrimers.
This confirmed that the synthesized nanomaterial was highly
porous and the reactant molecules could easily access the active
diamine moieties deep inside the dendrimers.
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), malononitrile
1 mmol) and solvent (2 mL).
(
These values are in good agreement with the data of the
standard Fe
the successful immobilization of basic moieties on the surface Hantzsch condensations was studied in detail. In the Knoeve-
of Fe @SiO @DSiO , no change in the crystalline structure nagel condensation, benzaldehyde and malononitrile were
Following complete characterization of the synthesized
3 4
O structure [JCPDS entry 19-0629]. Following nanomaterial, its catalytic activity for the Knoevenagel and
3
O
4
2
2
of magnetic nanoparticles could be observed as evident from selected as substrates for a model test reaction. In order to
Table 2 Optimization study for the one-pot synthesis of 1,4-polyhydroquinolines
Entry
Catalyst amount (mg)
Solvent
Temp (1C)
Time (min)
Isolated yield %
1
2
3
4
5
7
8
9
Fe
Fe
Fe
3
O
O
4
4
@SiO
@SiO
2
@DSiO
@DSiO
2
–NH
–NH
2
(10)
(10)
(10)
(10)
(15)
(10)
Ethanol
DCM
50
50
50
80
50
35
35
25
50
50
50
60
60
60
60
70
15
15
45
30
60
3
2
2
2
50
50
60
98
98
30
70
30
40
3
O
4
@SiO
2
@DSiO
2
–NH
2
CH
3
CN
Fe
Fe
Fe
3
O
4
O
4
O
4
@SiO
@SiO
@SiO
2
2
2
@DSiO
@DSiO
@DSiO
2
2
2
–NH
–NH
–NH
2
Toluene
3
2
H
H
H
H
2
2
2
2
O
O
O
O
3
2
Fe
Fe
3
O
4
@SiO
@SiO
Fe @SiO
Fe @SiO @DSiO
Fe @SiO (10)
Blank
2
@DSiO
@DSiO
–NH
2
–NH
2
(5)
3
O
4
2
2
–NH
2
(10)
10
11
12
13
3
O
4
2
2
(10)
Neat
3
O
4
2
2
(10)
H
H
H
2
2
2
O
O
O
120
120
120
3
O
4
2
30
Trace
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), dimedone (1 mmol), ammonium acetate (1 mmol), ethyl acetoacetate (1 mmol) and solvent (2 mL).
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3 4 2 2 2
Table 3 Fe O @SiO @DSiO –NH -catalysed Knoevenagel condensation
Entry
1
Aldehyde
Active methylene compound
Product
Time (min)
15
Isolated yield (%)
98
TON (TOF)
64 (256)
2
3
5
7
99
99
65 (778)
65 (554)
4
5
6
20
10
20
80
96
94
52 (157)
63 (376)
61 (184)
7
8
9
20
20
10
92
89
94
60 (180)
58 (174)
61 (368)
1
1
0
1
12
15
97
98
63 (317)
64 (256)
1
2
20
93
61 (182)
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Table 3 (continued)
Entry
Aldehyde
Active methylene compound
Product
Time (min)
12
Isolated yield (%)
96
TON (TOF)
63 (313)
1
3
1
1
4
5
22
20
91
90
59 (161)
59 (177)
1
6
21
92
60 (171)
Reaction conditions: aldehyde (1 mmol), active methylene compounds (1 mmol), water (2 mL), and catalyst (5 mg).
optimize the reaction conditions, the effect of various para- aldehydes having electron withdrawing groups resulted in
meters such as temperature, solvent and amount of loading of excellent yield of the product in lesser reaction time as com-
the catalyst were studied (Table 1). It was found that the yield of pared to the aldehydes having electron donating groups (which
3
the product decreased when DCM, ethanol, CH CN and THF took more time for the reaction to complete). In the case of
were used as solvents (Table 1, entries 2, 3 and –4). However, hydroxybenzaldehydes, the 4-substituted isomer was found to
excellent results could be obtained when water was used as a be more reactive as compared to the 2-substituted isomer due
solvent and the reaction was carried out at 25 1C. Furthermore, to steric hindrance in the case of the latter one (Table 3, entries
the yield of the product was found to be maximum when 5 mg 6 and 11). Heteroaromatic aldehydes were found to be less
catalyst was used and on decreasing the amount of catalyst to reactive towards the nucleophilic addition reaction (Table 3,
3
mg, the yield of the product reduced to 75%. In the absence of entry 4). Aliphatic aldehydes were also investigated but they are
the catalyst it could be said that only 10% transformation less reactive as compared to aromatic and heteroaromatic
occurred even at 40 1C in 40 minutes (Table 1, entry 11). There aldehydes (Table 3, entries 14, 15, and 16). In the case of
was no further increase in the yield of the product on increas- the aldehyde derivatives of pyridine, 2-substitued aldehyde
ing the amount of the catalyst (Table 1, entry 7). Hence, it could derivatives of pyridine were found to be more reactive than
be concluded that the maximum conversion could be obtained 4-substituted aldehyde derivative and this could be due to the
when the reaction was carried out with 5 mg of the catalyst and more pronounced inductive effect of the nitrogen atom at the
using water as a solvent at 25 1C (Table 1, entry 6).
ortho-position than at the para-position (Table 3, entries 7, and 8).
Similarly, in the case of the Hantzsch condensation,
Similarly, in the case of the Hantzsch condensation, alde-
4-nitrobenzaldehyde, dimedone, ammonium acetate and ethyl hydes with electron withdrawing groups (Table 4, entries 2,5
acetoacetate were taken as substrate molecules for the model and 9) as well as those with electron donating groups resulted
reaction and the optimization was done in a manner similar to in the excellent yields of the polyhydroquinolines (Table 4,
that in the case of the Knoevenagel condensation. It was found entries 3, 4 and 8). In the case of heteroaromatic aldehydes, like
that the yield of the product was maximum when the reaction pyridine-2-aldehyde and thiophen-2-aldehyde, the desired pro-
was carried out with 10 mg of the catalyst in water at 35 1C ducts could be obtained in good yields (90–94%). Furthermore,
(
Table 2, entry 7).
Using the optimized conditions, the catalytic efficiency of multicomponent reactions, aliphatic aldehydes were less reac-
the Fe O @SiO @DSiO –NH nanocatalyst was investigated tive as compared to aromatic aldehydes. Hence, the yield of the
as reported in the literature, it could be observed that in
3
4
2
2
2
1
for the Knoevenagel condensation (Table 3) as well as the product formed is lesser (Table 4, entries 12, and 13). The H
one-pot synthesis of 1,4-polyhydroquinolines (Table 4). Firstly, and C NMR spectra of the synthesized products are provided
1
3
in the Knoevenagel condensation it was found that aromatic in the ESI.† Based on these observations, it could be concluded
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3 4 2 2 2
Table 4 One-pot synthesis of polyhydroquinolines catalysed by Fe O @SiO @DSiO –NH
Entry
Aldehyde
Product
Time (min)
Isolated yield (%)
TON (TOF)
32 (95)
1
20
97
2
25
96
31 (75)
3
15
98
32 (128)
4
12
99
32 (162)
5
10
98
32 (192)
6
22
96
31 (85)
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Table 4 (continued)
Entry
Aldehyde
Product
Time (min)
Isolated yield (%)
TON (TOF)
30 (72)
7
25
92
8
9
22
20
20
22
95
97
94
98
31 (85)
32 (95)
31 (92)
32 (87)
1
1
0
1
1
1
2
3
25
22
90
91
29 (70)
30 (82)
Reaction conditions: aldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1 mmol), solvent (2 mL) and
catalyst (10 mg).
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Fig. 7 (a) Recyclability test for Fe
3
O
4
@SiO
2
@DSiO
2
–NH
2
in Knoevenagel and Hantzsch condensations. (b) TEM image of the recycled nanocatalyst after
nine runs. (c) FT-IR spectra of the recycled (after nine runs) and fresh nanocatalysts. (d) Magnetization curve of the recycled nanocatalyst after nine runs.
À1
that the Fe
3
O
4
@SiO
2
@DSiO
2
–NH
2
nanocatalyst showed excel- frequency of organosilane derivatives. Other peaks at 3342 cm
,
À1
À1
lent catalytic activity towards both Knoevenagel as well as 1056 cm and 453 cm correspond to hydroxyl groups, Si–O–Si
Hantzsch condensation. asymmetric stretching and Fe–O vibrations, respectively. This
Following completion of the reaction, the magnetic nanoca- confirms that the diamine moieties remain attached to the solid
talyst could be easily recovered from the reaction mixture by support material. No sign of destruction of organic moieties was
using an external magnet followed by washing with ethanol. observed after the recycling procedure. The magnetization curve
Since easy recyclability is stated to be a very important aspect of the recycled nanocatalyst after nine consecutive runs is
for a sustainable process, the magnetically recoverable nano- depicted in Fig. 7(d). From this curve it could be observed that
catalyst could be further used for nine consecutive cycles with- the saturation magnetization of the recycled nanocatalyst
À1
À1
out any significant loss in the catalytic activity (Fig. 7(a)). Slight decreases slightly i.e. from 17.20 emu g to 16.25 emu g . This
changes in the morphology of the recycled nanocatalyst due to confirms that the magnetization of the recycled nanocatalyst was
harsh reaction conditions in the nine consecutive cycles were maintained after catalyzing the reactions for nine consecutive
observed, as shown in Fig. 7(b). The FT-IR spectrum of the cycles. These consequences reveal that the synthesized nanocata-
recycled nanocatalyst after nine consecutive runs for synthesis lyst was well constructed and could tolerate the reaction condi-
of polyhydroquinolines is depicted in Fig. 7(c). It is observed tions. Hantzsch condensation requires harsh reaction conditions
À1
that the peak at 2929 cm corresponds to –CH2 stretching as compared to Knoevenagel condensation. Therefore, complete
Table 5 Comparative study of Fe
3 4 2 2 2
O @SiO @DSiO –NH with previously reported catalysts for Knoevenagel condensation using 4-nitrobenzaldehyde
and active methylene compounds
Entry
Catalyst
Catalyst amount
Time
Temperature (1C)
Solvent
Isolated yield (%)
Ref.
1
2
3
4
5
6
7
PMO-IL-NH
2
0.5 mol %
0.05 g
5 mg
20 mg
25 mg
20 mg
5 mg
90 min
10 min
10 min
5 h
6 h
100 min
5 min
RT
RT
75 1C
RT
40 1C
RT
25 8C
Solvent free
Ethanol
Water
Ethanol
Ethanol
Ethanol
Water
97
98
94
95
90
93
99
55
56
57
58
59
Fe
Fe
MSiO
Chitosan
WSiO –NH
@SiO @DSiO
3
O
4
@SiO
@SiO
–NH
2
-PVAm
3
O
4
2
-3N
2
2
2
2
60
Fe
3
O
4
2
2
–NH
2
This work
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Table 6 Comparative study of Fe
3
O
4
@SiO
2
@DSiO
2
–NH
2
with previously reported catalysts using 4-nitro benzaldehyde, active methylene compounds
4
and NH OAc
Entry
Catalyst
La
L-Proline
-imine/thiophene-Fe
Pd(0) NPs
Catalyst amount Time Temp (1C)
Solvent
Isolated yield (%)
Ref.
1
2
2
O
3
10 mol%
10 mol%
20 mg
0.04 Equiv.
0.03 g
85 min
6 h
60 min
4 h
40 min
25 min
35 min
25 min
60 min
15 min
10 min
RT
RT
100 1C
—
TFE
Ethanol (reflux)
Solvent-free
87
91
86
92
93
84
80
80
93
94
98
61
62
63
64
65
66
67
68
69
3
4
5
6
7
8
9
1
1
Ni(NO
3
)
2
3
O
4
@SiO
2
THF, reflux, Na
2
CO
3
, TBAB
Fe
3
O
4
@chitosan
MCM-41
/Cu@cellulose
RT
Ethanol
Solvent-free
Solvent-free
Solvent-free
Ethanol
1 mol%
3 mg
0.003 g
0.01 g
90 1C
RT
RT
g-Fe
O
2
O
3
2
À
Fe
Fe
3
4
@SiO
@SiO
Fe
@SiO
2
@PPh
@PPh
@PEO-SO
@DSiO
3
@Cr
2
O
7
Br]
3
O
4
2
3
@[CrO
3
RT
0
1
3
O
4
3
H
0.010 g
10 mg
RT
35 8C
Ethanol
70
Fe
3
O
4
2
2
–NH
2
H
2
O
This work
recycle-characterization was done for the Hantzsch condensation
to prove the stability of the synthesized nanocatalyst.
It was also found that Fe O @SiO @DSiO –NH was superior
3 4 2 2 2
in comparison to other previously reported catalysts for the one-
In order to investigate the efficacy of this catalyst, the results pot synthesis of polyhydroquinolines. In the case of some of the
obtained in Knoevenagel and Hantzsch condensations were catalysts there were certain limitations encountered during the
compared with those of the previously reported catalysts (Tables 5 extraction of product from the reaction mixture, while in the case
and 6). It was found that as compared to the previously reported of some other catalysts, there was difficulty faced in the separation
3 4 2 2 2
systems the catalytic efficiency of Fe O @SiO @DSiO –NH was of the catalysts (Table 6, entries 2, and 6) or they required longer
superior in terms of shorter reaction time, recyclability, tempera- reaction times, high amount of catalyst loading and higher
ture and excellent yield in green solvents. Hence, it could be said reaction temperatures (Table 6, entries 3, 7, and 8) and often
that the diamine-functionalized magnetic core–shell dendritic resulted in lower yields of products or required the use of organic
3 4 2 2 2
mesoporous silica (Fe O @SiO @DSiO –NH ) could be used as solvents (Table 6 entries 1, 4 and 5).
a highly efficient and cost-effective catalyst for both Knoevenagel
and Hantzsch condensations.
Finally, in order to explore the leaching of the catalyst in the
reaction mixture during the progress of Knoevenagel condensation,
Fig. 8 Proposed reaction mechanism for (a) Knoevenagel condensation and (b) Hantzsch condensation.
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a hot filtration test was carried out with 4-nitrobenzaldehyde Ultimately, adduct (1) and intermediate (II) could generate inter-
and malononitrile as substrates for the model reaction. When mediate (III) via a Michael addition reaction to form intermediate
half of the reaction was completed, the catalyst was magneti- (III), which could result in the formation of the desired product
cally separated from the reaction mixture and the reaction was through intramolecular cyclization and dehydration.
allowed to continue for a considerable amount of time without
the catalyst. It was found that, no further progress could be
6
. Conclusion
observed in the reaction. Similarly, for Hantzsch condensation
-nitrobenzaldehyde, dimedone, ethyl acetoacetate and ammo-
4
In conclusion, the present work describes the successful synthesis
of Fe O @SiO @DSiO –NH nanospheres (with assured core–shell
nanostructures and accessible dendritic mesoporous channels) by
combining the co-precipitation, sol–gel approach and an oil–water
biphasic stratification-coating strategy. The as-synthesized Fe
SiO @DSiO –NH nanocatalyst displayed excellent catalytic activity
nium acetate are chosen as model substrates and the same
procedure was repeated as before for the Knoevenagel conden-
sation. From these observations, it was confirmed that there
was no leaching of diamine groups from the nanocatalyst i.e.
Fe O @SiO @DSiO –NH during the progress of the reaction.
3
4
2
2
2
3 4
O @
3
4
2
2
2
2
2
2
for both Knoevenagel condensation and the one-pot synthesis of
polyhydroquinolines. This could be due to the easy accessibility of
active diamine sites and super paramagnetic nature of the prepared
nanocatalyst. The prepared catalyst could be utilized for multiple
catalytic cycles without any notable loss in the activity.
5
. Plausible mechanism
5.1. Knoevenagel condensation
A mechanism can be proposed for the synthesis of the Knoevenagel
product through the nucleophile addition reaction using Fe O @
3
4
SiO @DSiO –NH as a heterogeneous base catalyst (Fig. 8(a)). In
2
2
2
Conflicts of interest
the first step, the –NH group could abstract the proton from the
2
There are no conflicts to declare.
active methylene compound. This could lead to the generation
of negative charge over the active methylene compound, which
could act as a nucleophile. Subsequently, the nucleophilic
attack of this nucleophile on carbonyl carbon could lead to
Acknowledgements
the formation of a b-hydroxyl compound (the Knoevenagel Author Surabhi gratefully acknowledges the University Grant
adduct) and finally the catalyst could be regenerated. This Commission, New Delhi, India for financial support. The
Knoevenagel adduct could ultimately lead to the formation of authors also thank the USIC-CIF, University of Delhi, Delhi,
the product by the elimination of a molecule of water.
India for the instrumental facilities, and SAIF-AIIMS Delhi and
SAIF IIT ROORKEE for the instrumentation facilities.
5.2. Hantzsch condensation
The multicomponent synthesis of biologically active 1,4-
polyhydroquinolines proceeds through different mechanistic
pathways to the conventional ones. The synthesized nanocata-
lyst possesses magnetic core–shell dendritic mesoporous silica
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